The present invention relates to an optical beam scanning system.
There are known optical systems employing a light deflector for scanning a light beam. For utilization of a scanned light beam in a greater range without physical interference with a light beam applied to the light deflector, it has been the general practice to apply the light beam to the light deflector in a direction toward the axis of rotation of the light deflector and at a finite angle, except normal incidence, to a plane lying perpendicularly to the axis of rotation of the light deflector.
For example, diverging rays of light emitted from a semiconductor laser are rendered parallel by a collimator lens, and then converged by a scanning lens positioned in front of a light deflector comprising a rotating polygonal mirror. The converging light beam from the scanning lens falls on reflecting surfaces of the light deflector which extend parallel to the axis of rotation thereof, such that the principal ray of the light beam is applied at an angle .theta. to a plane lying perpendicularly to the axis of rotation of the light deflector.
Upon rotation of the light deflector, the light beam reflected therefrom is progressively deflected to scan a scanned object such as a photosensitive drum along its generatrices while forming an image thereon.
The scanning light applied to the photosensitive drum travels in a direction at the angle .theta. to the plane normal to the axis of rotation of the light deflector. Therefore, the light beam applied to the light deflector and the light beam reflected therefrom are angularly spaced 2.theta. out of physical interference with each other.
With the conventional optical beam scanner, as shown in FIG. 18 the collimator lens 1 and the light from the semiconductor laser are disposed on a straight line aligned with the path of travel of the principal ray immediately before it is applied to the reflecting surfaces of the light deflector. This arrangement requires that the semiconductor laser and the photosensitive drum be spaced a certain distance from each other along the axis of rotation of the light deflector, resulting in an obstacle to efforts to reduce the size of the optical scanning system.
One solution is to fold the optical path with a prism and a mirror, thereby reducing the distance between the semiconductor laser and the photosensitive drum. However, the inclusion of these prism and mirror makes the scanner structurally complex and increases the cost of the scanner.